<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
      div.parent{
          width: 200px;
          height: 200px;
          background:blue;
      }
      /* opacity设置元素的透明度 */
      div.child{
          width: 100px;
          height: 100px;
          background: red;
          opacity: 0.3;
      }
    </style>
</head>
<body>
    <div class="parent">
        <div class="child">
        </div>
    </div>
</body>
</html>